This invention relates to a method for compensating for the transfer characteristic of a printing device used in a halftoning process for image screening. Background for this invention may be found in the publications "PostScript Language Reference Manual, Second Edition" by Adobe Systems Incorporated (Addison-Wesley 1990) and "PostScript Screening: Adobe Accurate Screen" by Peter Fink (Adobe Press 1992).
Screening is the method used to reproduce continuous tone images and tints using technologies or media that can only represent "on" and "off" states, usually by picture elements ("pixels"). The regions of the image having continuous tones are broken into small areas or cells. The "shade" of each such area is represented by a predetermined set of device pixels such that the percentage area covered by the device pixel set equals the correct shade of gray when viewed from a distance. Although the term "gray-scale" and "gray level values" are used throughout this specification, this by no means implies that the invention is limited to monochromatic images. The gray level values, or shades of gray, may be the shades of density of a particular color in a color image. Color images are represented by the percentages of each of the color primaries making up that color, and each such primary usually has its own individual gray level.
Traditional AM screening uses variable-size halftone dots at fixed spacing in both dimensions. The size of the dot is increased by adding device pixels at its outer edge to increase the covered area. When viewed from a distance, the larger the dot size, the greater the area covered and the darker the image area. Traditional screening has been termed "AM" or amplitude modulated screening, since the variable being changed was dot size (amplitude).
More recently, as computation power has increased in screening or printing devices, AM screening is being replaced by frequency-modulated, or "FM" screening. FM screening uses a fixed-size, smaller dot at variable spacing to achieve the same effect as traditional AM screening. Variation in dot spacing varies the number of dots in a given area, or dot frequency, hence the term FM screening. The denser the dot distribution (meaning the dots are closer together), the darker the image area. On some output devices, each dot for FM screening is actually made up of four or more device pixels. FM screening provides a dot distribution based upon the shade variations in the original image. The dot distribution is optimized to be the best representation possible for the particular output device or system. The screening is not constrained to the coarser fixed grid dot pattern used in AM screening.
The benefits of FM screening are dramatic for color images where three or four primary printing colors are overlaid. FM screening more closely represents the original image, especially those images with a lot of detail.
The halftoning process is used to convert image data from a requested multilevel gray or color value into a printable pattern of bi-level or multi-level gray or color values which typically takes the form of a "printable pattern". If this printable pattern is to be made up of a series of binary pixels, then the pattern is called a "bit pattern", since a bit is binary and a binary pixel can only be either "on" or "off". Each bit represents a pixel of ink on the printed page.
If there are no bits to be printed, the page will be all white. If all the bits are to be printed, the page will be the maximum color density in the case of color, or black in the case of monochrome printing. If half the bits are turned on, the printed area should look mid-gray (or an in-between color density).
However, printable patterns can be made up of data which is not binary, where gray-scale pixels are either fully on, fully off or somewhere in between. In such cases, since screens are still made up of pixels, a dot in the printable pattern is still made up of a plurality of pixels. Stochastic screening, which is a combination of AM and FM screening, uses both variable dot size as well as variable spacing. Some stochastic approaches use adaptive algorithms to determine the best combination of dot size and placement to most faithfully reproduce the image.
Unfortunately, while the above seems simple, and should result in faithful reproduction of an image onto a printed page, in the real world, that is not the case. There is seldom a truly linear relationship between the number of bits turned on and printed in an area of the page and the actual, colorimetrically measured color value for that area. With most laser printers, printing presses and imagesetters, for example, the actual printed area is darker that it should be when the binary pixels to be printed are based solely upon the percentage of printed pixels calculated directly from the percentage gray level value. This increase in darkness has been called "dot gain". With a given fraction of bits turned on, the printed and measured gray level and the dot gain depends upon the actual bit pattern used. Dot gain is the smallest if the turned on bits are grouped in large clusters. It is worst if the printed pixels are separated by non-printed pixels. Since FM screening takes advantage of pixel separation, dot gain has become a more severe problem as FM screening has replaced AM screening in high quality printing processes. For example, an actual measured gray level of 91% has resulted from printing an area with only 50% of the dots turned on.
Dot gain is affected by many parameters. They include, but are not limited to, printing device resolution, device pixel size, printing technology (such as xerography vs. webb offset), ink characteristics, the paper used (SWOP-coated vs. newsprint) and the plate-making process. For any given bit pattern, the dot gain will vary widely across the range of parameters depending upon these variables.
Various prior art methods have been applied to deal with the dot gain problem. One such method uses the transfer function in the Adobe PostScript.TM. interpreter, used to generate characters and screens in many high end laser printers, imagesetters and other offset printers. This transfer function maps 256 incoming color levels to 256 outgoing color levels. While this works satisfactorily for moderate dot gains, its fails to compensate adequately for severe dot gains, because many incoming gray levels are forced to be mapped to the same outgoing gray levels. If compensation requires a shift up, for example, then the lower of the 256 outgoing levels cannot be used. This reduces the number of distinctly printable gray levels, sometimes by as much as 50%. Where the original incoming print data may have had eight bits of accuracy, when mapped onto fewer than the full 256 outgoing color levels, printing is no longer carried out with eight bits of accuracy. The result is poorer print quality with shade steps within the printed area.
With big dots, a dot gain of 20%, for example, is not too serious. However, as dots get smaller, as they do in FM screening, and where dot gains may reach 50%, the print accuracy is much more substantially and noticeably reduced from the original data with 8-bit accuracy.
Accordingly, it would be highly desirable to be able to compensate for dot gain in a screening process in a manner which did not appreciably reduce the faithfulness of the reproduction of the data being printed.